On-fiber tunable coupler and high speed switch for telecommunication applications

ABSTRACT

An on-fiber tunable coupler and switch for communication systems. Two optical fibers are modified by replacing passive cladding with at least an inner metallic electrode, an electrooptic material layer on the inner electrode, and an outer metallic electrode on the electrooptic material. The optical fibers are positioned close to each other. More layers can be coated between any of these layers to improve the interface properties or reflectivity. The presence of an electric signal, will change the optical properties of the modified cladding, as will the propagation characteristics of the optical signal within one fiber core. This will permit tunable coupling of the signal from the first fiber core to the second fiber core(s). The coupling coefficient between the first and the second fiber(s) is adjustable. Switch-off the signal output from the first fiber to the second fiber is achieved at 100% coupling ratio.

FIELD OF THE INVENTION

This invention relates to an on-fiber tunable coupler and switchcomprising two optical fibers having modified cladding. Moreparticularly, invention relates to a coupler and switch where thecoupling coefficient is adjustable by an external electric controlsignal.

BACKGROUND OF THE INVENTION

Because of the importance of integrated optics in the telecommunicationindustry, a great deal of effort has been expended to reduce insertionloss associated with coupling a light between integrated opticalcomponents and fiber links, to the lowest possible level and withreasonable expenses. Such components are on-fiber modulators, switches,couplers, and distribution devices. There also exists a need for a fiberoptic coupler which is tunable for high accuracy, inexpensive, easilyfabricated and one which can be used without the problems of couplingthe light signal from or to the optical link. This invention is animprovement on U.S. Pat. No. 5,060,307.

have invented such an on-fiber tunable coupler which uses two opticalfibers, each having a small region from which the cladding material hasbeen removed. The fibers are then placed parallel and close to eachother before they are coated with an active multilayer materialsstructure in the region of uncladded fibers. The multilayer coating isconstructed of at least three layers, a conductive layer coated on thesurface of the fiber cores, an electrooptic material coated on the topof the conductive layer (the inner electrode) and a conductive layercoated on the top of the electrooptic material acting as the outerelectrode. The advantage of this multilayer coating is that it issensitive to electromagnetic fields and will satisfy the condition thatin the presence of an external electromagnetic field, the opticalproperties of the modified region will change.

Another advantage is achieved by tuning the optical properties of theelectrooptic material which is controlled by applying an externalelectromagnetic field to the device across the region of coating withthe active multilayer materials. The device of this invention can beused as an electromagnetic field sensor for determination of the appliedexternal signal. The active medium or the coated materials in the regionof the unclad fibers controls energy transferred between the fibers andprovides modulation of the signal transmitted in each optical fiber whenan external signal is applied.

Other advantages will appear hereinafter.

SUMMARY OF THE INVENTION

It has now been discovered that the above and other advantages of thepresent invention may be achieved in the following manner. The inventionis directed to the combination of an external signal applied to twomodified optical fibers, wherein the optical fibers having a regionwhere the cladding material has been removed and replaced with an activemultilayer materials system, where the two modified optical fibers arepositioned in parallel to each other to provide close proximity betweenthe fibers at the area where the cladding material has been to produce amodified fibers interaction region.

This invention is directed to an apparatus for coupling optical signalfrom one fiber to another, switching signal between two fibers ortransmitting and distributing an optical signal propagating in anoptical fiber. The optical fibers comprises a core surrounded by apassive cladding layer in which each optical fiber has the passivecladding material removed to produce an uncladded section of the fiberor fibers. An active medium comprising of least three layers of activematerials is placed on the fiber core. These layers are the innermetallic electrode (or the first electrode) coated on the surface of theoptical fiber core, the electrooptic material layer coated on the top ofthe first electrode, and the outer metallic electrode (or the secondelectrode) coated on the top of the electrooptic material.

The invention includes a light source means for launching opticalsignals to the input of the first optical fiber and an external electricsignal means applied to the active multilayer material system fortunable coupling of optical signal from said first optical fiber tosecond optical fiber or for switching off the light signal transmittedthrough the fiber. Also included are means for detecting, signalprocessing, and reading out the light signal transmitted in each opticalfiber; and referenced signal means for noise signal elimination andsignal stability.

This invention is also directed to a method for modulating, switching,coupling or distributing a light signal being transmitted through anoptical fiber consisting of a transparent core surrounded by a claddinglayer comprising applying an external signal to a section of the opticalfiber which has had the cladding material removed and said section iscoated with metallic conductive material and immersed in an activemedium comprising an electrooptic active material, such as, liquidcrystal or electro-optic polymer or solid state material.

This invention is directed to the device which uses as the appliedsignal, a signal such as an electrical, magnetic, electromagnetic ormicrowave applied to the region of uncladding and to a device usingsolutions having a large electro-optic and/or magneto-optic effect aspart of the active medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The invention described herein is shown by reference to the drawings,although it is to be understood that the drawings are referred to onlyfor purposes of illustration and example, and the scope of the inventionis not limited thereto. For a more complete understanding of theinvention, reference is hereby made to the drawings, in which:

FIG. 1 shows a schematic view of the optical system incorporating atunable fiber optic coupler and switch;

FIG. 2 shows a schematic view of an apparatus of a high speed (2×2) or(1×2) tunable optical coupler and switch;

FIG. 3 shows a twin core fiber with identical or two different coreindices as an alternate embodiment of the two parallel fibers; and

FIG. 4 shows a two dimensional coupler and switch (5×5) or (n×n) arrayof all-fiber structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an optical fiber 1A with a portion of the cladding materialremoved and replaced with modified cladding 4. A light signal from thelight source 11 transmitted in the optical fiber is modulated by anexternal signal applied by means of connections and two electrodes tothe region of modified section and detected by photodetector 12 andprocessed by means of a signal processing means 15. Device 10 is a highspeed tunable coupler and/or switch.

In FIG. 1, the continuous changing of the applied externalelectromagnetic field results in continuous changing of the opticalproperties of the modified section which provides optical intensitymodulation of the propagating optical signal. Also, the continuouschanging of the applied external electromagnetic field results incontinuous changing of the coupling coefficient between the two opticalfibers. The optical signal coupled from said first optical fiber to saidsecond optical fiber is detected by a photodetector 12 and processed bymeans of a signal processing means 15. The output 16 of the firstoptical fiber and the output 18 of the coupled signal to the secondfiber can be read out. A reference signal 13 is used to enhance theoutput and eliminate noise.

The moved components of this invention, shown in FIG. 1, can be used asa switch. Switching-off the optical signal in the optical fiber occurswhen the applied signal changes the optical properties of the modifiedsection to certain higher values. Also, switching-off the optical signalin the optical fiber occurs when the applied signal changes the phase ofthe propagating signal by a half wavelength or 180° with respect to theoriginal reference signal. FIG. 2 is a (2×2) or (1×2) tunable opticalcoupler and switch. FIG. 2 shows two optical fibers 1A and 1B. Opticalfiber 1A has had a portion of the fiber cladding 3 removed and replacedwith a multilayer modified cladding 4,5, and 6. Optical fiber 1B, also,has had a portion of the fiber cladding 3 removed and replaced withmodified cladding 4,5, and 6. An optical signal is transmitted intooptical fiber 1A through the optical signal input 7. The signal in fiber1A is coupled partially or totally to fiber 1B by an external couplingsignal applied by means of electrode 5 and 6 to the region ofmodification. The second electrode 6 is grounded 8. The electrode isused to modulate the optical properties of the modified region. Couplingof the optical signal between fibers 1A and 1B is modulated, i.e.tunable coupling can be achieved. The interacting regions of the twoadjacent fibers are coated with at least a three layer structure, innerelectrode 5, electrooptic material 4, and outer (or second) electrode 6.FIG. 2 shows the path of the optical signal through fiber core (singlemode or multimode) 2 of optical fiber 1A, passing through the modifiedcladding and then coupled into fiber core 2 of optical fiber 1B. Thefigure shows the path that the optical fiber takes as it passes from thefirst fiber to the second fiber. The optical signal propagating in thefiber core 2 is partially (or totally for switches) coupled to themodified cladding where the material properties are modulated by theexternal signal. At the end of the modified region, the optical signalis distributed between optical fiber 1A and optical fiber 1B. Theremaining optical signal in fiber 1A exits fiber 1A through output 17and the coupled optical signal exits optical fiber 1B through the signaloutput 18.

FIG. 3 is an alternate embodiment of FIG. 2. FIG. 3 is a high speed(2×2) or (1×2) tunable optical coupler and/or switch. FIG. 3 shows atwin core fiber with identical or two different core indices and/ordiameter as an alternate embodiment of the two parallel fibers.Therefore, the two parallel fibers used in FIG. 2 can be replaced in asmall region by a single twin core fiber. The passive cladding can bereplaced in a small region by the same multilayer structure described inFIG. 2, where external electromagnetic field can be applied to controlthe coupling coefficient between the two fiber cores. An optical signalis transmitted into one of the two optical fiber cores through theoptical signal input 7. The signal is coupled to the second fiber coreusing an external microwave, radio frequency, or direct voltage. FIG. 3shows the path of the optical signal input 7 through the first fibercore (single mode or multimode) 2, passing through the modified claddingand then passing into the second fiber core 2. The figure shows the paththat the optical fiber takes as it passes from the first fiber core tothe second fiber core. The optical signal propagating in the first fibercore 2 is partially coupled to the modified cladding where the materialproperties are modulated by the external signal applied to theelectrodes 5 and 6. At the end of the modified region, the coupledoptical signal can be detected and transmitted. The modulated opticalsignal exits the first fiber core through output 17 and exits the secondfiber core through the coupled signal output 18.

FIG. 4 shows an all-fiber two dimensional coupler and switch (5×5) or(n×n) array. Each element 10 of the two-dimensional coupler or switch isa (2×2) tunable coupler 10 that could be one of the (2×2) couplers shownin FIGS. 2 and 3. In FIG. 4, a set of input channels (a1–a5) called (a)channels input and a set of input channels (b1–b5) are mixed in an arrayfashion at the input of 5×5 tunable coupler units 10. The signal atevery channel of the 5×5 coupler output is the combination of a.bsignals, and can be controlled by controlling the applied voltage toeach individual coupler 10.

While particular embodiments of the present invention have beenillustrated and described, it is not intended to limit the invention toany specific embodiment except as defined by the following claims.

1. Apparatus selectively operable as a two dimensional array of couplersor switches wherein each element of the array is an on-fiber coupler,said elements are arranged in a series and parallel connections adaptedto couple the input signal of any element to the output communicationline of any other element in the array, wherein each element is atunable coupler comprising: a first optical fiber and a second opticalfiber, each of said fibers having a core and a cladding materialsurrounding the core, and each of said fibers having a region whereinthe cladding has been removed to form a region of uncladding, saidregions of uncladding being positioned parallel and close to each otherwhile preventing mechanical movement of said fibers in relation to eachother; each fiber core is coated, in said region of uncladding, all overthe 360° of the cylindrical surface, with a thin layer of metallicmaterial to construct the inner (or first) electrode on each fiber core;a layer of an electrooptic material is coated on the tope of each inner(first) electrode and around the two fibers as well as filling the gapbetween the fibers; on the top of the electrooptic material, a layer ofmetallic material is coated allover the surface (360°) to construct theouter (or second) electrode, the inner and outer electrodes are used toapply electromagnetic fields to the electrooptic material in a symmetricradial directions with respect to the two fiber cores; the field linesare oriented in a symmetric configuration with respect to the fiberscircular cross section in order to provide a polarization independentcoupling; means for applying an external electric signal to saidmodified fibers to change said optical properties of the modified regionto transfer partially or totally the optical signal from the firstoptical fiber to the second optical fiber; a light source means focusedto provide an optical communication signal into said first optical fiberand a reference signal; detector means positioned on said each opticalfiber downstream from said light source and said region of modifiedfiber for detecting optical signals in said optical fiber and saidreference signal and transfer optical signals to electrical signals; andsignal processing means coupled with said detectors means to amplify andprocess any output signals of said detectors, wherein said referencesignal from light source is used to provide a noise free coupleroutputs.
 2. The apparatus of claim 1, which further includes elementshaving additional coating layers between each two layers of those coatedmaterials to improve the interface and the structure properties.
 3. Theapparatus of claim 1, operable as a two dimensional array of tunablecouplers, wherein said means for applying an external electric signal toeach element of the array includes means for applying externalelectromagnetic fields to provide a polarization independent controlsignal to said modified regions to tune the coupling ratio between saidfirst and second fibers, whereby an optical signal from said lightsource in said first fiber is partially transferred with an exact ratioto said second fiber.
 4. The apparatus of claim 1, operable as a twodimensional array of switches, wherein said means for applying anexternal electric signal to each element of the array includes means forapplying an external electromagnetic field to provide a sufficientcontinuous field to said modified region in order to switch off signalsin first fiber of a specific element of the array.
 5. The apparatus ofclaim 1, operable to twin core optical fibers, wherein each element ofthe array is a twin core fiber, wherein said modification is applied toa small region of the fiber, wherein tunable coupling is formed betweenthe fiber twin cores.
 6. Apparatus selectively operable as a twodimensional array of couplers or switches wherein each element of thearray is an on-fiber coupler, said elements are arranged in a series andparallel connections adapted to couple the input signal of any elementto the output communication line of any other element in the array,wherein each element is a tunable coupler comprising: a first opticalfiber and a second optical fiber, each of said fibers having a core anda cladding material surrounding the core, and each of said fibers havinga region wherein the cladding has been removed to form a region ofuncladding, said regions of uncladding being positioned parallel andclose to each other while preventing mechanical movement of said fibersin relation to each other; each fiber core is coated, in said region ofuncladding, all over the 360° of the cylindrical surface, with a thinlayer of metallic material to construct the inner (or first) electrodeon each fiber core; a layer of an electrooptic material is coated on thetope of each inner (first) electrode and around the two fibers as wellas filling the gap between the fibers; on the top of the electroopticmaterial, a layer of metallic material is coated allover the surface(360°) to construct the outer (or second) electrode, the inner and outerelectrodes are used to apply electromagnetic fields to the electroopticmaterial in a symmetric radial directions with respect to the two fibercores; the field lines are oriented in a symmetric configuration withrespect to the fibers circular cross section in order to provide apolarization independent coupling; a power supply for applying anexternal direct electric signal to said inner and outer electrodes tochange the optical properties of the modified region to transferpartially or totally the optical signal from the first optical fiber tothe second optical fiber; a light source means focused to provide anoptical communication signal into said first optical fiber; detectormeans positioned on said each optical fiber downstream from said lightsource and said region of modified fibers for detecting optical signalsin said optical fiber and said reference signal and transfer opticalsignals to electrical signals; and signal processing means coupled withsaid detectors means to amplify and process any output signals of saiddetectors, wherein said reference signal from light source is used toprovide a noise free coupler outputs.
 7. The apparatus of claim 6, whichfurther includes elements having additional coating layers between eachtwo layers of those coated materials to improve the interface and thestructure properties.
 8. The apparatus of claim 6, operable as a twodimensional array of tunable couplers, wherein said power supply forapplying an external direct electric signal to each element of the arrayincludes source for applying external electromagnetic fields to providea polarization independent control signal to said modified regions totune the coupling ratio between said first and second fibers, whereby anoptical signal from said light source in said first fiber is partiallytransferred with an exact ratio to said second fiber.
 9. The apparatusof claim 6, operable as a two dimensional array of switches, whereinsaid power supply for applying an external electric signal to eachelement of the array includes source for applying an externalelectromagnetic field to provide a sufficient continuous field to saidmodified region in order to switch off signals in first fiber of aspecific element of the array.
 10. The apparatus of claim 6, operable totwin core optical fibers, wherein each element of the array is a twincore fiber, wherein said modification is applied to a small region ofthe fiber, wherein tunable coupling is formed between the fiber twincores.